Visualizing Charge Carrier Dynamics in Transition Metal Dichalcogenide Nanoflakes Using Femtosecond Pump-Probe Microscopy

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Nontechnical description: Semiconductor nanostructures are playing an ever-increasing role in technologies that range from solar energy conversion to light emitting diodes. These nanostructures are small crystals, perhaps 1/1000 the diameter of a human hair, and come in a variety of shapes, including small particles, long flexible wires, or thin sheets. They are not perfect crystals, however. Atoms at the edges and surfaces act differently from those located in the interior. In addition, their small dimensions increase their flexibility, enabling wires to bend, and sheets to buckle and fold, which in turn changes their properties. Nanostructure functionality is oftentimes dominated by these imperfections, yet it is difficult to disentangle the influence of specific defect structures on material behavior. This project is addressing this challenge by developing microscopy methods that can image the movement of electrons in individual nanostructures on short time scales, and with high spatial resolution. These methods are then used to explore the spatial variation in electronic dynamics across semiconductor nanostructures that are only a few atomic layers thick. The project's discoveries could have broad implications for emerging technologies ranging from solar energy conversion and catalysis to quantum information systems. The research also provides training opportunities for graduate and undergraduate students, and the project participants are working with college science majors from underrepresented backgrounds to develop skills in scientific communication. Technical description: The project is advancing pump-probe microscopy methods and using them to explore how the electronic relaxation dynamics and charge carrier transport vary spatially across semiconductor nanoflakes. The nanoflakes are a few microns across and consist of 3-30 transition metal dichalcogenide layers (like sheets of paper) held together by weak van der Waals forces. Individual nanoflakes are excited with a femtosecond laser pulse that is focused to a diffraction limited spot by a microscope objective, promoting electrons from the valence band to the conduction band in a localized region of the structure. The excited electron population is then probed by a second focused laser pulse that is delayed in time. The probe pulse ejects electrons from the structure through a multiphoton absorption process, and their momentum is measured using velocity-map imaging photoelectron spectroscopy. The momentum distribution of the ejected electrons provides a direct window through which the energetic relaxation can be monitored. Experiments performed at different locations in the nanoflake (e.g. edge vs. center) show how different structural features affect the charge carrier relaxation and recombination. The microscope can also excite the nanoflake in one location, and probe it in another, enabling direct visualization of the electrons as they move through the structure. The microscopy tools developed by the research activities are broadly applicable to a wide range of the nanomaterials, and the students are gaining experience in the construction of sophisticated instrumentation. The graduate and undergraduate students are also working with entering freshmen in the University of North Carolina Chancellor's Science Scholars Program to explore the importance and challenges of explaining complex scientific discoveries to broad audiences through the development of short videos that highlight the project's research results.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.